There are today a variety of types of information mediums having an information layer, which is irradiated with an optical beam so that prescribed information is recorded on the information layer and/or information recorded on the information layer is reproduced. The variety of types of information mediums are different in thickness thereof and/or in material of the information layer and/or in structure of the information layer. These variety of types of information mediums are all disc-shaped (hereinafter, these information mediums will be referred to as “discs”), and can be exchangeably used in common recording and reproduction apparatuses.
These discs include recordable discs usable for information recording and reproduction, represented by, for example, DVD-RAM discs and DVD-R discs, and reproduction-only discs only usable for information reproduction, represented by DVD-ROM discs. In order to record information on or reproduce information from a recordable disc, or in order to reproduce information from a reproduction-only disc, it is necessary to position an optical beam to the center of a track provided on a disc by tracking control.
FIG. 14 is a block diagram illustrating a principle of tracking control of a conventional recording and reproduction apparatus. A disc 31, on which the recording and reproduction apparatus records information or from which the recording and reproduction apparatus reproduces information, includes a substrate 32. An information layer 33 used for information recording and reproduction is formed on the substrate 32. The recording and reproduction apparatus includes an optical pickup 1 provided so as to face the information layer 33 formed in the disc 31. The optical pickup 1 includes an objective lens and a photodetector having two light receiving sections. The objective lens provided in the optical pickup 1 collects an optical beam onto the information layer 33. The photodetector receives the optical beam reflected by the information layer 33, converts the received optical beam into a tracking signal, and outputs the tracking signal to a tracking error detection circuit 11. Based on the tracking signal converted by the photodetector provided in the optical pickup 1, the tracking error detection circuit 11 detects a tracking error signal representing an error, along a radial direction of the information medium 31, between the position of the optical beam collected onto the information layer 33 and the central position of the track formed on the information layer 33. Then, the tracking error detection circuit 11 outputs the tracking error signal to a tracking control circuit 13. Based on the tracking error signal detected by the tracking error detection circuit 11, the tracking control circuit 13 generates a tracking driving signal for performing phase compensation, such that the position of the optical beam collected onto the information layer 33 tracks a control target position representing the central position of the track formed on the information layer 33. Then, the tracking control circuit 13 outputs the tracking driving signal to a tracking driving circuit 16. Based on the tracking driving signal generated by the tracking control circuit 13, the tracking driving circuit 16 controls the position of the objective lens provided in the optical pickup 1 such that the position of the optical beam collected onto the information layer 33 tracks the central position of the track.
Many systems by which the tracking error detection circuit 11 detects a tracking error signal have been proposed and put into practice. A representative system for detecting a tracking error signal from an optical beam reflected by a recordable disc usable for information recording and reproduction is a push-pull system. Hereinafter, the push-pull system will be described.
According to the push-pull system, a difference, in light intensity of an optical beam received by the two light receiving sections which are located so as to be symmetrical with respect to the central position of the track formed on the information layer 33, is detected as a tracking error signal.
The light intensity of the optical beam reflected by the information layer 33 significantly relies on the depth of a groove of the track formed in the information layer 33 or the depth of a pit formed on the information layer 33.
The information layer 33 has, for example, a concaved portion (hereinafter, referred to as a “groove portion”) which is a spiral groove formed therein and a convexed portion (hereinafter, referred to as a “land portion”) between the concaved portions. An optical path length of the optical beam reflected by the groove portion is longer than an optical path length of the optical beam reflected by the land portion by twice the depth of the groove. Therefore, the waveform of the optical beam reflected by the groove portion and the waveform of the optical beam reflected by the land portion have a phase difference corresponding to a length which is twice the depth of the groove.
FIG. 15 is a graph illustrating the relationship between the depth of the groove of the track formed in the information layer 33 and the intensity of the tracking signal converted from the optical beam reflected by the information layer 33. The horizontal axis represents the depth of the groove portion provided in the information layer 33, and λ represents the wavelength of the optical beam, which is directed to the information layer 33 by the optical pickup 1 (or with which the optical pickup 1 irradiates the information layer 33). The vertical axis represents the intensity of the tracking signal detected by the photodetector provided in the optical pickup 1.
When the depth of the groove portion provided in the information layer 33 of the disc 31 is λ/4, the optical path length of the optical beam reflected by the groove portion is longer than the optical path length of the optical beam reflected by the land portion by λ/2, which is twice as long as the depth of the groove, i.e., λ/4. Accordingly, the waveform of the optical beam reflected by the groove portion and the waveform of the optical beam reflected by the land portion have a phase difference of π/2 corresponding to λ/2, which is twice the depth of the groove. Therefore, the waveform of the optical beam reflected by the groove portion and the waveform of the optical beam reflected by the land portion counteract each other. As a result, the intensity of the tracking signal detected by the photodetector provided in the optical pickup 1 is minimum as shown in FIG. 15.
When the depth of the groove is λ/8, the optical path length of the optical beam reflected by the groove portion is longer than the optical path length of the optical beam reflected by the land portion by λ/4, which is twice as long as the depth of the groove, i.e., λ/8. Accordingly, a phase difference of π/4 corresponding to λ/4, which is twice the depth of the groove, is caused. At this point, the intensity of the tracking signal detected by the photodetector provided in the optical pickup 1 is maximum as shown in FIG. 15. When the depth of the groove is λ/8 to λ/6, the intensity of the tracking signal does not significantly decrease from the maximum intensity obtained when the depth of the groove is λ/8. This is why the depth of the groove portion provided in a DVD-R disc, which is a recordable disc usable for information recording and reproduction, is set to be equal to or more than λ/8 and equal to or less than λ/6.
When the center of the optical axis of the objective lens, of the optical pickup 1, for collecting the optical beam onto the information layer 33 is shifted from the border between the two light receiving sections provided in the photodetector (hereinafter, this shifting will be referred to as a “lens optical axis shifting”), the intensity of the optical beam received by one of the two light receiving sections is higher than the intensity of the optical beam received by the other of the two light receiving sections. As a result, a DC offset is superimposed on a tracking error signal detected by the tracking error detection circuit 11.
FIG. 16 illustrates the positional relationship between the center of the optical axis of an optical beam and the two light receiving sections provided in the photodetector. In the photodetector 10 provided in the optical pickup 1 (FIG. 14), the two light receiving sections a and b are located so as to be in contact with each other and so as to be symmetrical with respect to the central position of the track formed on the information layer 33. The objective lens 5 provided in the optical pickup 1 (FIG. 14) is located such that an optical axis center A thereof matches the border between the light receiving sections a and b.
When the objective lens 5 is shifted from the position represented by the solid line to the position represented by the dashed line along a radial direction of the disc 31, the optical axis center A of the objective lens 5 is shifted toward the light receiving section b, to an optical axis center B, by distanced. Therefore, the center of the optical axis of the optical beam, which is incident on the photodetector 10 after being reflected and diffracted by the information layer 33 of the disc 31, is shifted toward the light receiving section b by distance d. As a result, the amount of light of the optical beam which is incident on the light receiving section a provided in the photodetector 10 is smaller than the amount of light of the optical beam which is incident on the light receiving section b. Thus, the amount of light incident on the light receiving section a and the amount of light incident on the light receiving section b becomes out of balance.
As described above, according to the push-pull system, a difference in the light intensity between the optical beams respectively received by the two light receiving sections is detected as a tracking error signal. Therefore, when the amounts of light incident on the two light receiving sections become out of balance due to the shifting of the center of the optical axis of the optical beam incident on the photodetector 10, a DC offset is superimposed on the tracking error signal. In a recordable disc such as a DVD-R disc or the like, the depth of the groove is set to be equal to or more than λ/8 and equal to or less than λ/6 such that the intensity of the tracking signal is not significantly reduced. Therefore, the amount of the DC offset superimposed on the tracking error signal is increased in the recordable disc.
When the DC offset is superimposed on the tracking error signal as described above, even when the optical beam is controlled based on the tracking error signal such that the position on the information layer 33 to which the optical beam is directed matches the center of the track, the actual position on the information layer 33 to which the optical beam is directed is shifted from the center of the track. Therefore, a system for compensating for the DC offset superimposed on the tracking error signal has been proposed.
FIG. 17 is a block diagram illustrating a principle of conventional tracking control having a function of compensating for a DC offset. Identical elements previously discussed with respect to FIG. 14 bear identical reference numerals and the detailed descriptions thereof will be omitted. The apparatus shown in FIG. 17 is different from the recording and reproduction apparatus described above with reference to FIG. 14 in that the apparatus shown in FIG. 17 further includes an optical axis shifting amount estimation circuit 14, a multiplication circuit 15 and a switching circuit SW1.
The optical axis shifting amount estimation circuit 14 generates a signal representing an optical axis shifting amount estimation value, which is an estimated value of the lens optical axis shifting amount, based on the tracking driving signal generated by the tracking control circuit 13. Then, the optical axis shifting amount estimation circuit 14 outputs the generated signal to the multiplication circuit 15. The multiplication circuit 15 multiplies the signal representing the optical axis shifting amount estimation value, which is output from the optical axis shifting amount estimation circuit 14, with a compensation gain, and then outputs the resultant signal representing a DC offset estimation amount to the switching circuit SW1. When the switch circuiting SW1 is turned on, the signal representing the DC offset estimation amount, which is output from the multiplication circuit 15, is added to the tracking error signal detected by the tracking error detection circuit 11. The DC offset superimposed on the tracking error signal is compensated for by the signal representing the DC offset estimation amount. When the switching circuit SW1 is turned off, the signal representing the DC offset estimation amount is not added to the tracking error signal. Thus, the switching circuit SW1 is structured so as to be capable of turning on or off a negative feedback based on the DC offset estimation amount.
The compensation gain, multiplied by the multiplication circuit 15 with the signal representing the optical axis shifting amount estimation value, is determined based on a measured DC offset which is superimposed on the tracking error signal. Hereinafter, a method for measuring a DC offset for determining the compensation gain and a method for determining the compensation gain based on the measured DC offset will be described.
FIG. 18 shows a structure of a conventional recording and reproduction apparatus 90. Identical elements to those of the recording and reproduction apparatus previously discussed with respect to FIG. 17 bear identical reference numerals and the detailed descriptions thereof will be omitted. The recording and reproduction apparatus 90 includes an optical pickup 1.
The optical pickup 1 is placed on a transportation table 2. The transportation table 2 having the optical pickup 1 placed thereon transports the optical pickup 1 along a radial direction of an information medium 31 based on an instruction from a system controller 17. In this manner, the optical pickup 1 is transported by the transportation table 2 and thus moved to an arbitrary position along the radial direction of the information medium 31, where the optical pickup 1 can operate to record information on or reproduce information from the information layer 33.
The optical pickup 1 has a light source 7. The light source 7 is formed of a red semiconductor laser device. The light source 7 oscillates an optical beam having a wavelength of 650 nanometers (nm) and emits the optical beam toward a collimator lens 8. The optical beam emitted from the light source 7 (hereinafter, referred also to as “emitted light”) is converted into parallel light by the collimator lens 8, passes through a beam splitter 9, is converged by an objective lens 5, and is directed to the information layer 33 of the disc 31.
The optical beam reflected by the information layer 33 passes through the objective lens 5 and the beam splitter 9 and is incident on a photodetector 10 having two light receiving sections. The photodetector 10 outputs, as a tracking signal, a difference in the light intensity between the optical beams respectively received by the two light receiving sections to a tracking error detection circuit 11.
The optical pickup 1 includes a tracking actuator 6. Based on a driving current from a tracking driving circuit 16, the tracking actuator 6 moves the objective lens 5 with respect to the transportation table 2 along the radial direction of the information medium 31.
The tracking error detection circuit 11 detects a tracking error signal by the above-described push-pull system from the tracking signal output from the photodetector 10, and outputs the tracking error signal to an offset measuring circuit 3 and an offset subtraction circuit 12. The offset subtraction circuit 12 subtracts a signal representing a DC offset estimation amount from the tracking error signal detected by the tracking error detection circuit 11, and outputs the resultant compensated tracking error signal to a tracking control circuit 13. The DC offset estimation amount will be described later in detail.
Based on the compensated tracking error signal output from the offset subtraction circuit 12, the tracking control circuit 13 generates a tracking driving signal for performing phase compensation such that the position of the optical beam collected onto the information layer 33 tracks a control target position representing the central position of the track formed on the information layer 33. Then, the tracking control circuit 13 outputs the tracking driving signal to a switching circuit SW2.
In response to an instruction from the system controller 17, the switching circuit SW2 selects either a tracking driving signal output from the tracking control circuit 13 or a lens shift driving signal output from a lens shift driving circuit 18, and outputs the selected signal to the tracking driving circuit 16. For causing the position of the optical beam collected onto the information layer 33 to track the control target position, representing the central position of the track formed on the information layer 33 by tracking control, the switching circuit SW2 selects the tracking driving signal output from the tracking control circuit 13, and outputs the tracking driving signal to the tracking driving circuit 16. For measuring a DC offset for determining the compensation gain, or for transporting the optical pickup 1 to an arbitrary position along the radial direction of the information medium 31 by the transportation table 2, the switching circuit SW2 selects the lens shift driving signal output from the lens shift driving circuit 18, and outputs the lens shift driving signal to the tracking driving circuit 16. The lens shift driving signal output from the lens shift driving circuit 18 will be described below with reference to FIG. 21A.
In accordance with the tracking driving signal or the lens shift driving signal output from the switching circuit SW2, the tracking driving circuit 16 outputs a driving current, for moving the objective lens 5, to the tracking actuator 6. Based on the driving current from the tracking driving circuit 16, the tracking actuator 6 moves the objective lens 5 with respect to the transportation table 2 along the radial direction of the information medium 31.
Next, the DC offset estimation amount will be described in detail. The DC offset estimation amount is an estimated value of the DC offset which is superimposed on the tracking error signal when lens optical axis shifting occurs to the objective lens 5. The DC offset estimation amount is obtained as follows.
The offset measuring circuit 3 detects a maximum value and a minimum value of the tracking error signal detected by the tracking error detection circuit 11. The offset measuring circuit 3 calculates the difference between the detected maximum and minimum values so as to measure the DC offset superimposed on the tracking error signal. Then, the offset measuring circuit 3 outputs the DC offset to a compensation gain determination circuit 4. Based on the DC offset measured by the offset measuring circuit 3, the compensation gain determination circuit 4 determines the compensation gain and outputs the compensation gain to a multiplication circuit 15.
The tracking control circuit 13 outputs, as a tracking correction signal, a low frequency component of the tracking driving signal to an optical axis shifting amount estimation circuit 14. The optical axis shifting amount estimation circuit 14 has a dynamic characteristic which is equal to a dynamic characteristic of the objective lens 5, which operates in accordance with the output from the tracking actuator 6. Based on the tracking correction signal output from the tracking control circuit 13, the optical axis shifting amount estimation circuit 14 generates a signal representing an optical axis shifting estimation value, which indicates a displacement substantially equal to a displacement caused by the lens optical axis shifting of the objective lens 5 driven by the tracking actuator 6. Then, the optical axis shifting amount estimation circuit 14 outputs the generated signal to the multiplication circuit 15.
The multiplication circuit 15 multiplies the compensation gain determined by the compensation gain determination circuit 4 with the signal representing the optical axis shifting estimation value generated by the optical axis shifting amount estimation circuit 14. Then, the multiplication circuit 15 outputs the resultant signal representing a DC offset estimation amount to the switching circuit SW1. The switching circuit SW1 is turned on or off in response to an instruction from the system controller 17. When the switching circuit SW1 is turned on, the switching circuit SW1 supplies the signal representing the DC offset estimation amount, output from the multiplication circuit 15, to the offset subtraction circuit 12.
With reference to FIGS. 19 and 20, a conventional method for measuring a DC offset will be described. FIG. 19 illustrates a principle of conventional offset measuring method, and FIG. 20 is a flowchart illustrating the principle of the conventional offset measuring method. In FIG. 19, the disc 31 facing the optical pickup 1 placed on the transportation table 2 in each of steps 91, 93 and 95 is not shown, but the optical pickup 1 actually faces the disc 31.
It is assumed that the switching circuit SW2 selects the lens shift driving signal output from the lens shift driving circuit 18, and the lens shift driving signal is input to the tracking driving circuit 16.
The optical pickup 1 having the objective lens 5 is placed on a neutral position of the transportation table 2. The optical pickup 1 having the objective lens 5 is moved by distance X1 with respect to the transportation table 2 in one radial direction of the disc 31 (for example, toward an outer periphery of the disc 31) by the tracking actuator 6, the tracking actuator 6 receiving the driving signal from the tracking driving circuit 16 in accordance with the lens shift driving signal. The optical beam, directed by the optical pickup 1 which has moved by distance X1 in the one radial direction and reflected by the information layer 33, is converted into a tracking signal by the photodetector 10 provided in the optical pickup 1. The tracking error detection circuit 11 detects a tracking error signal from the tracking signal converted by the photodetector 10. The offset measuring circuit 3 measures a DC offset based on the tracking error signal detected by the tracking error detection circuit 11 (step 91).
Next, the optical pickup 1 having the objective lens 5 is driven by the tracking actuator 6 so as to move by distance 2×X1 in the other radiation direction (for example, toward an inner periphery of the disc 31). The optical pickup 1 is moved from the position which is away from the neutral position by distance X1 in the one radial direction, to the position which is away from the neutral position by distance X1 in the other radial direction. The DC offset is measured in the above-mentioned manner based on an optical beam directed by the optical pickup 1 which has moved by 2×distance X1 in the other radial direction and reflected by the information layer 33 (step 93). Then, the optical pickup 1 having the objective lens 5 is moved to the neutral position (step 95). As described above, the two positions of the optical pickup 1 on the transportation table 2 for measuring the DC offset are symmetrical with respect to the neutral position.
With reference to FIGS. 21A through 21C and 22, a method for determining a compensation gain based on the measured DC offset will be described. FIG. 21A is a graph illustrating the relationship between the lens shift driving signal and time according to the conventional method. FIG. 21B is a graph illustrating the relationship between the position of the objective lens 5 and time according to the conventional method. FIG. 21C is a graph illustrating the relationship between the tracking error signal and time according to the conventional method. FIG. 22 is a flowchart illustrating a procedure of the conventional offset measuring method. Identical elements to those of the flowchart previously discussed with respect to FIG. 20 bear identical reference numerals and the detailed descriptions thereof will be omitted.
The optical beam directed by the optical pickup 1 is positioned on the track provided on the information layer 33 of the disc 31. The photodetector 10 is assumed to be in a state capable of receiving the optical beam reflected by the information layer 33 and converting the optical beam to a tracking signal.
First, as shown in FIG. 21A, in response to an instruction from the system controller 17, the lens shift driving circuit 18 outputs a lens shift driving signal, for moving the objective lens 5 provided in the optical pickup 1 by distance X1 at a constant speed in one radial direction, to the tracking driving circuit 16 via the switching circuit SW2. Based on the lens shift driving signal output from the lens shift driving circuit 18, the tracking driving circuit 16 outputs a driving current for driving the tracking actuator 6. As shown in FIG. 21B, in accordance with the driving current output from the tracking driving circuit 16, the tracking actuator 6 moves the objective lens 5 by distance X1 in the one radial direction from the neutral position on the transportation table 2 at a constant speed (step 91).
When the objective lens 5 is moved by distance X1 from the neutral position on the transportation table 2 in the one radial direction as described above, a DC offset OS11 caused by a lens optical axis shifting is superimposed on the tracking error signal detected by the tracking error detection circuit 11 as shown in FIG. 21C. The offset measuring circuit 3 measures a maximum value and a minimum value of the tracking error signal detected by the tracking error detection circuit 11 and obtains the DC offset OS11 based on the measured maximum and minimum values (step 92).
Next, as shown in FIG. 21A, the lens shift driving circuit 18 outputs a lens shift driving signal, for moving the objective lens 5 by distance 2×distance X1 at a constant speed in the other radial direction, to the tracking driving circuit 16 via the switching circuit SW2. Based on the lens shift driving signal, the tracking driving circuit 16 outputs a driving current for driving the tracking actuator 6. In accordance with the driving current output from the tracking driving circuit 16, as shown in FIG. 21B, the tracking actuator 6 moves the objective lens 5 in the other radial direction at a constant speed, to a position away from the neutral position on the transportation table 2 by distance X1 in the other radial direction (step 93).
When the objective lens 5 is moved to the position away from the neutral position on the transportation table 2 by distance X1 in the other radial direction as described above, a DC offset OS12 having an opposite characteristic to that of the DC offset OS11 is superimposed on the tracking error signal detected by the tracking error detection circuit 11 as shown in FIG. 21C. The offset measuring circuit 3 measures a maximum value and a minimum value of the tracking error signal detected by the tracking error detection circuit 11 and obtains the DC offset OS12 based on the measured maximum and minimum values (step 94).
Then, as shown in FIG. 21A, the lens shift driving circuit 18 outputs a lens shift driving signal, for moving the objective lens 5 in the one radial direction at a constant speed to the initial position before step 91 was performed, to the tracking driving circuit 16 via the switching circuit SW2. Based on the lens shift driving signal, the tracking driving circuit 16 outputs a driving current for driving the tracking actuator 6. In accordance with the driving current output from the tracking driving circuit 16, as shown in FIG. 21B, the tracking actuator 6 moves the objective lens 5 to the initial position (step 95).
The compensation gain determination circuit 4 determines a compensation gain based on the DC offset OS11 measured in step 92, the DC offset OS12 measured in step 94, the distance 2×X1 (=X1+X1) by which the objective lens 5 was moved in step 93, and expression (1) shown below (step 96).Compensation gain={OS11+OS12}/{X1+X1}  expression (1)
Then, in response to an instruction from the system controller 17, the switching circuit SW2 selects the tracking driving signal output from the tracking control circuit 13, and outputs the tracking driving signal to the tracking driving circuit 16. Based on the tracking driving signal, the tracking driving circuit 16 outputs a driving current for driving the tracking actuator 6. In accordance with the driving current output from the tracking driving circuit 16, the tracking actuator 6 drives the objective lens 5. As a result, the optical beam converged by the objective lens 5 and directed to the information layer 33 of the disc 31 is positioned in the vicinity of the center of the track provided on the information layer 33.
Next, by an instruction from the system controller 17, the switching circuit SW1 is turned on, and the signal representing the DC offset estimation amount, which is output from the multiplication circuit 15, is input to the offset subtraction circuit 12. The offset subtraction circuit 12 subtracts the signal representing the DC offset estimation amount from the tracking error signal, and outputs the resultant compensated tracking signal to the tracking control circuit 13. As a result, the control target value in tracking control becomes a corrected value which is obtained by subtracting the DC offset superimposed on the tracking error signal. Therefore, the optical beam converged by the objective lens 5 and directed to the information layer 33 is positioned on the substantial center of the track provided on the information layer 33.
Thus, even when the lens optical axis shifting occurs to the objective lens 5 for some reason and as a result, a DC offset is superimposed on the tracking error signal, the optical beam converged by the objective lens 5 and directed to the information layer 33 of the disc 31 can be positioned to the substantial center of the track provided on the information layer 33, as long as the switching circuit SW1 is closed. Therefore, recording of an information signal on, or reproduction of an information signal from the information layer 33, can be stably performed.
However, the above-described conventional method for measuring the DC offset has the following problem. As shown in FIG. 19, the position on the information layer 33 to which the optical beam is directed after being emitted from the optical pickup 1, for the purpose of measuring a DC offset for the first time in step 91, is different from the position on the information layer 33 to which the optical beam is directed after being emitted from the optical pickup 1 for the purpose of measuring a DC offset for the second time in step 93. Therefore, when the information layer 33 has different reflection characteristics or different transmission characteristics at these two different positions, the DC offset may not be measured with high precision.
For example, when the information layer 33 has dirt, dust or an unrecoverable defect (hereinafter, referred to as a “defect”) at either one of the position on the information layer 33 to which the optical beam is directed after being emitted from the optical pickup 1 for the purpose of measuring a DC offset for the first time in step 91, or the position on the information layer 33 to which the optical beam is directed after being emitted from the optical pickup 1 for the purpose of measuring a DC offset for the second time in step 93, the optical beam reflected at the position having the defect is fluctuated by the influence of the defect as well as by the influence of the lens optical axis shifting. Therefore, signals other than a DC offset signal, generated by the influence of the lens axis shifting, are superimposed on the tracking error signal detected by the tracking error detection circuit 11. As a result, the DC offset cannot be measured with high precision.
There is another problem as follows when a plurality of different types of discs are exchangeably used for a common recording and reproduction apparatus. Until the information recorded on the disc mounted on the recording and reproduction apparatus is reproduced, it cannot be determined whether or not the area of the disc to be used for information recording or reproduction is an area where the DC offset should be compensated for by a signal representing a DC offset estimation amount.
The present invention for solving these problems has an objective of providing an offset measuring method and a recording and reproduction apparatus for measuring, with high precision, a DC offset superimposed on a tracking error signal caused by lens optical axis shifting.
Another objective of the present invention is to provide an offset measuring method and a recording and reproduction apparatus for determining an area on a disc where a DC offset superimposed on a tracking error signal should be compensated for by a signal representing a DC offset estimation amount.